This invention relates generally to circuits for measuring current and, more particularly, circuits to sense when a current falls outside a predetermined range.
In many instances, it is desirable to know that the level of a current is within acceptable limits. One important instance where the value of a current is very important is in a ground fault interrupt (GFI) circuit. These circuits are used to reduce the chance that a person will receive a serious electrical shock from an electrical appliance dropped into water or otherwise malfunctioning.
A GFI circuit operates by sensing the current on the power line leading up to the appliance. The power line has two wires. When the electrical appliance is operating correctly, the current in each wire is equal since all the current supplied in one of the wires returns in the other wire.
If some of the current supplied to the electrical appliance is flowing out of the appliance, a dangerous condition exists. For example, when an appliance drops into water, current may flow out of the appliance and shock a person. To avoid this unpleasant result, GFI circuits sense that more current is flowing into the electrical appliance than is flowing back from the appliance. Then the GFI circuit disconnects all power from the electrical appliance.
Circuits to sense currents are commercially available. For example, National Semiconductor manufactures and sells an integrated circuit chip designated LM1851. The circuit is designed to be connected to the secondary of a transformer. The primary of the transformer is made from both wires of the power line to the appliance. If there is an imbalance in the current in the two wires, there will be a current induced in the secondary winding.
For the transformer to accurately reflect the imbalance of current in the primary to the secondary, the impedance in the secondary must be very low. Thus, the impedance of the circuit to sense currents must be very low, typically less than 100 ohms. Also, the circuit to sense current must be able to tell when the magnitude of the current exceeds a threshold, regardless of the direction of the current.
While the LM1851 performs adequately, it is desirable to have a circuit which is as simple as possible to sense current. A simpler circuit is less expensive to fabricate and less likely to fail.
Also, the LM1851 contains an amplifier with zener diodes in a feedback loop. The zener diodes convert the current at the input of the LM1851 to a voltage for sensing. If the voltage exceeds a threshold, a fault condition is indicated. However, because the LM1851 operates by converting a current to a voltage, its supply voltage must be above a certain level. It would be desirable in some circumstances to build a current sense circuit to operate at lower voltages, say as low as 3 volts or less.